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Measles --- United States, 2004

Measles is a highly infectious, acute viral illness that can cause severe pneumonia, diarrhea, encephalitis, and death.
During 2004, a total of 37 cases (incidence: <1 case per million population) was reported to CDC by local and state
health departments, the lowest number of measles cases ever reported in 1 year in the United States and a decrease of 16% from
the previous low of 44 cases in 2002 (1). This report describes the epidemiology of measles in the United States in
2004, documenting the absence of endemic measles and the continued risk for internationally imported measles cases that can
result in indigenous transmission.

Case Characteristics

Of the 37 cases, 34 (92%) were confirmed by laboratory testing (i.e., detection of measles-specific IgM antibodies
or measles virus) and the remaining three (8%) were confirmed by meeting the clinical case definition
(2) and by being epidemiologically linked to a laboratory-confirmed case. Confirmed measles cases occurred predominantly among
preschool-aged children (aged 1--4 years), with 18 cases (49%), followed by children aged 5--19 years, with seven cases (19%),
and persons aged 20--34 years and infants aged <12 months, with five cases each (14%); two cases occurred in persons aged
>35 years. Three states accounted for 49% of cases: Washington (seven cases), California (six cases), and New York (five
cases, including four from New York City); 11 other states
reported one to three cases. No cases were reported during 32 of the
52 reporting weeks; 12 consecutive weeks was the longest period during which no cases were reported (Figure). The
maximum number of reported cases occurring during a single week was four, and the median number of cases per week was one
(range: zero to four cases).

Twenty-seven (73%) of the 37 cases were imported*; 14 (52%) cases occurred in U.S. residents who acquired measles
while traveling abroad, and 13 (48%) occurred in foreign
nationals who acquired disease abroad and traveled to the United
States. The countries from which measles was imported were China (13 cases), India (four), Bangladesh (two), and Thailand
(two), with six other countries contributing one case each (Malaysia, Nigeria, Philippines, Russia, Saudi Arabia, and the
United Kingdom). Of the 27 persons with imported measles cases, 13 (48%) were infectious during aircraft flights (i.e., rash
onset occurred within 4 days before through 4 days after the date of arrival). One case of transmission after exposure on an
aircraft flight was documented in a passenger who had been vaccinated with 2 doses of measles-containing vaccine and who was
seated next to a person with infectious disease. All 14 U.S. residents with imported cases were eligible for measles
vaccination, according to recommendations from the Advisory Committee on Immunization Practices
(3). Of these, nine (64%) were unvaccinated, three (21%) had
unknown vaccination status, and two (14%) had been vaccinated with
>1 dose of measles-containing vaccine. Of the
13 imported cases among non-U.S. residents, 10 (77%) were in unvaccinated persons and
three (23%) were in persons with unknown vaccination
status.

Ten (27%) of the cases were
indigenous, of which six (60%) were import-linked and four (40%) had unknown sources
of exposure (two occurring in a two-case chain of transmission and two sporadic cases with no epidemiologic link to any
other measles case). Eight (80%) cases occurred in vaccine-eligible persons (i.e., aged
>12 months and born after 1957); of
these, five (63%) persons were unvaccinated, one (13%) had
unknown vaccination status, and two (25%) had been
vaccinated.

Outbreaks

During 2004, two measles outbreaks, defined as three or more epidemiologically linked cases, were reported to CDC.
These outbreaks occurred in five states and accounted for
13 (35%) of the 37 cases. In one outbreak, nine children aged
12--18 months who acquired disease while in orphanages in China traveled as adoptees to three states (Maryland, New York,
and Washington). One case of secondary spread was identified in a California resident aged 19 years with a nonmedical
exemption for measles vaccination who had had close contact with one of the adoptees
(4). In the second outbreak, a U.S. student
aged 19 years with a nonmedical exemption for measles vaccination was infected in India and returned to Iowa, where
two
secondary cases occurred: one in an unvaccinated close contact of the index patient and one in a person
who had been seated next to the index patient on an aircraft
(5).

Viral Genotypes

Three genotypes of measles virus were identified among
viral samples collected from nine patients. D8, a genotype found
in South Asia, was identified from cases in the outbreak arising from the U.S. traveler returning from India, a two-case chain
of transmission resulting from travel of the index patient from India, and a single case imported from Bangladesh. Genotype
H1, endemic in East Asia, was detected from cases in the outbreak traced to adoptees from China and from an unrelated
two-case chain of transmission involving an adoptee from China. Virus isolated from a single case
imported from the Philippines was determined to belong to genotype D3.

Editorial Note:

The 37 confirmed cases in 2004 represent a record low number of reported measles cases since
measles became a nationally reportable disease in 1912. The epidemiology of measles in 2004 confirms the previous finding
that endemic transmission of measles virus has been eliminated in the United States
(6). Thirty-three (89%) cases were import-associated (i.e., imported or import-linked), and 14 imported cases occurred among U.S. residents who contracted
measles while traveling abroad. Sixty-four percent of the imported cases among U.S. residents could have been prevented if
long-standing ACIP recommendations concerning measles vaccination of foreign travelers
(3) had been followed.

Of the 27 persons with imported cases in 2004, 13 (48%) traveled on aircraft while infectious. Measles virus is a
highly infectious pathogen, and intercontinental flights create the potential for prolonged exposure. However, on the basis
of available data, the risk for in-flight measles transmission among passengers appears to be low
(7). Of the hundreds of persons on the same flights as the 13 persons who traveled while
infectious in 2004, only one case of secondary transmission
was identified, in a person seated immediately next to an
infectious passenger. For the 8-year period (1996--2004) for which
such transmission data have been recorded, 117 passengers with imported measles cases were considered infectious while
traveling by aircraft (carrying an estimated 10,000 passengers), but only four secondary-spread cases were identified from three
index patients (CDC, unpublished data,1996--2004). Seating location was recorded for two of the three index patients, both
of whom were seated immediately adjacent to the secondary-spread patients. The low in-flight attack rate might be related
to high vaccination/immunity levels among persons traveling by air (most of whom are adults) and to vertical airflow
patterns within airplanes, which might decrease in-flight exposure to measles.

As long as measles is endemic in most countries worldwide, sustaining measles elimination in the United States
will require maintenance of high levels of vaccination coverage (i.e., >90%)
(8), vigilance in detecting and containing imported
cases, and enhanced surveillance to detect and characterize
cases and identify sources and viral genotypes.

Acknowledgments

This report is based, in part, on data contributed by state and local health departments.

* Imported cases are those in persons infected outside the United States.

 Indigenous cases are those in persons infected in the United States. Indigenous cases are classified into three groups: import-linked (i.e., epidemiologically linked
to an imported case); imported virus (i.e., cases that cannot be linked epidemiologically to an imported case but for which imported virus has been isolated from
the patient or from an epidemiologically linked patient); and unknown source (i.e., all other cases acquired in the United States for which no epidemiologic link
or
virologic evidence indicates importation).

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